Basically, carbohydrates break down into sugar. Some break down quickly and some don't. It's the slow-breaking down ones which are the best and are needed to keep the blood sugar levels stable. It's the glycaemic index (how quickly the food is converted to blood sugar) that needs to be considered.
http://www.diabetes.org/food-and-fi...arbohydrates/glycemic-index-and-diabetes.html
People without diabetes can get a "sugar rush" or a sugar low without ill-effect, but diabetics need to keep things stable and constant and not have highs and lows. They need to eat so that their blood sugar stays as stable as possible.